The present invention relates to a ghost canceling circuit which is installed in a television receiver.
Generally, a television receiver receives through an antenna a television signal composed of a directly received signal and delay signals which reach the antenna through several reflecting routes in which the signals are reflected by neighboring geographical features, neighboring buildings, moving vehicles or the like. Due to such delay signals, generally, multiple images may be come out to some extent on the received picture. Such delay signals causing the multiple images are called "a ghost" and a phenomenon that large multiple images deteriorate the quality of the picture is called "a ghost obstacle".
A ghost canceling apparatus, used for canceling such ghosts, is provided with a dummy-ghost producing unit consisting of a transversal filter and a ghost detecting unit. The ghost detection unit detects the condition of the occurrence of the ghost that varies with time on the basis of a reference waveform which has been inserted at a predetermined position of the received television signal, and automatically controls a tap gain which is to be supplied to the transversal filter. Further, this ghost canceling apparatus is provided with an adder unit for composing (i.e., adding) the television signal originally received and a dummy ghost produced from this television signal.
The dummy-ghost producing unit arranged by the transversal filter produces the dummy ghost by using a group of cascaded delaying units, a group of coefficients and an adder, which dummy ghost simulates a mechanism of ghost occurrence such as delay of the signal, damping, and inter-summing that are caused by multiple reflection. This dummy ghost is generally produced in opposite polarity and is added through the adder unit to the television signal originally received, thereby canceling out with the ghost components contained in the television signal.
The afore-mentioned ghost may be divided into two main groups for convenience, a nearby ghost that appears substantially at the same time as the original signal (includes a case in which the ghost may be somewhat prior to the television signal) and a normal ghost that appears about several micro seconds after the signal originally received.
In other words, as shown in FIG. 1, assuming that a waveform A shown in a dotted line represents the signal originally received when no ghost exists, the nearby ghost causes a distorted waveform a as shown in a solid line and non-nearby ghost produces a distorted waveform b. The non-nearby ghost appears in the time axis away from the signal originally received and tends to repeatedly appear due to multiple reflection in the order of a parent ghost, a child ghost, and then a grandchild ghost with a certain time space therebetween, gradually being damped. Thus to eliminate the non-nearby ghost it is desirable to apply a cyclic composition to both the television signal originally received and the dummy ghost produced from the television signal originally received. In the mean time, the nearby ghost overlaps the original television signal on the time axis, thus the produced nearby dummy-ghost may have a portion ahead of the original signal on the time axis, therefore the cyclic type composition cannot be applied to the produced ghost and the original signal.
Additionally, with the nearby ghost, the waveform distortion due to the ghost and the waveform distortion due to transmission characteristics occur in such a way that they are integral and inseparable; therefore cancellation of the nearby ghost can be thought of as a kind of waveform equalizing.
Thus, one method that has been proposed is a method in which cancellation of the nearby ghost and the non-nearby ghost is performed in two steps, i.e., the nearby ghost is first canceled by cascading exclusive canceling apparatuses but not a single apparatus and then the non-nearby ghost. The entire ghost canceling apparatus is arranged, as shown in FIG. 2, by a nearby ghost processing unit connected in cascade with a normal ghost processing unit, the former consisting of a nearby ghost cancellation unit 101a and a nearby ghost detection unit 101b and the latter consisting of a normal ghost cancellation unit 102a and a normal ghost detection unit 102b.
While the nearby ghost is a special phenomenon that cannot be clearly distinguished from various transmission factors that may cause the waveform distortion, the non-nearby ghost is originated from a phenomenon of detoured propagation path formation specific to the ghost. Therefore, the non-nearby ghost is often referred to as a normal ghost. This non-nearby ghost is referred to as the normal ghost in the following description.
Such a ghost canceling apparatus requires multiple averaging processes, several tens to several hundred times, of the reference waveform extracted from respective frames in the received television signal, and therefore suffers from a problem that ghost detection takes time. With a ghost canceling apparatus shown in FIG. 2, the detection and cancellation of the normal ghost is initiated after the detection and cancellation of the nearby ghost has been completed thus the entire processing takes time, thereby being unable to follow the ghost that varies rapidly with passage of, for example, moving objects.
Also, the normal ghost is detected and canceled on the basis of the signal after the nearby ghost has been canceled thus detection characteristics of the normal ghost varies depending on the conditions of cancellation of the nearby ghost.